1. Field of the Invention
The present invention relates generally to rotational motion sensor, such as gyroscopes, and more specifically, but not exclusively, to micromachined, mems-based, sensing vibrating gyroscopes for measuring angular velocity and, optionally, acceleration.
2. Description of Related Art
Vibrating gyroscopes rely on the vibration of a proof mass in one direction and in detecting the Coriolis force generated in a perpendicular direction by the rotational speed. Vibrating gyroscopes are formed, for example, by etching a semiconductor to form a proof mass suspended by a spring system, such as elastic beams, to the substrate. An electronic drive circuit which may be on the same substrate applies an alternating drive current to driving electrodes which vibrate the proof mass in a drive direction. The sensor further comprises sensing electrodes for detecting displacements of the proof mass in a sensing direction orthogonal to the drive direction. Those displacements may be caused by a Coriolis force when an angular velocity is applied to the gyroscope, and used for measuring this velocity. Accelerations applied to the sensor along the sense axis may be measured, in a different frequency band, with the same sensing electrodes.
The production process and the technology used for producing the springs and the beams in mems (micro electrical mechanical systems)-based gyroscopes often lead to quadrature errors, i.e. errors caused by driving the vibrating proof mass along a direction which is not exactly perpendicular to the direction along which the Coriolis movement is measured. The component of the vibrating drive movement in the sense direction generates an output signal (the quadrature signal) superposed to the signal caused by the Coriolis force or by the acceleration.
The quadrature output signal is in phase with the drive signal used for driving the proof mass, while the component of this output signal due to the Coriolis force is phase shifted. Electronic demodulation circuits are thus known in the art for separating those two components. An accurate separation is however difficult especially when the amplitude of the quadrature signal is large compared with the Coriolis sense signal.
Additional electrodes or mechanical trimming for reducing the quadrature forces and the quadrature signal have been suggested. Those additional components and trimming processes add to the complexity and cost of the system.
U.S. Pat. No. 6,571,630 B1 uses material ablation or deposition to reduce quadrature forces. U.S. Pat. No. 7,051,590 B1 describes an example of quadrature nulling electronic circuit that measures quadrature errors and applies corresponding compensation. Such a quadrature nulling circuit is difficult to build, and often introduces problems of phase uncertainty.
WO-A1-03/010492 describes a quadrature nulling method in which quadrature servo applies sinusoidal forces to a sensing element. This method uses a proof mass with a scalloped edge for modulating mechanically the DC signal from the quadrature servo. This solution avoids the problems of phase uncertainty in AC servo signals, but still requires an electronic quadrature servo circuit for actively compensating the quadrature forces.
It is therefore an aim of the invention to propose a new gyroscope in which the quadrature errors and the quadrature signal are reduced.
It is another aim of the invention to propose a new gyroscope in which the quadrature errors and the quadrature signal may be reduced even when no quadrature servo circuit is used, or when the quadrature signal is unknown.
In the present document, a quadrature force designates any force along the sense axis in phase with the drive signal oscillation. A quadrature signal designates a component or part of the output signal caused by those quadrature forces. Quadrature signals designate in particular detrimental signals which are superposed to the output sense signal and which are caused by unwanted displacements of the proof mass along the sense axis when the proof mass is vibrated. In this application, a neutral point designates a point in the proof mass such that a rotation around an axis passing through the neutral point will essentially not lead to a change in the capacitive output signal. For example, in the case of a flat proof mass with constant thickness, the neutral point corresponds to the center of gravity of the proof mass.